This application is related to Japanese application No. Hei 10(1998)-340902 filed on Dec. 1, 1998, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a reflective liquid crystal display, and, in particular to a reverse mode polymer-dispersed liquid crystal display, wherein the display comprises a display medium made of a liquid crystal and a liquid crystalline polymer, molecules of the liquid crystalline polymer and liquid crystal aligned in the same orientation to form a network structure, and the display can transmit light when no voltage is applied thereto and can scatter light when a voltage is applied thereto.
2. Description of the Related Art
Recently, as a result of social needs for energy saving, low power consumption is required of display devices, too. This trend is also applied to liquid crystal display devices which are inherently non-light emitting type and consume less power as compared with CRT or the like. Therefore, development works have been increasingly active with reflective color liquid crystal display devices in order to achieve much more reduced power consumption.
However, the reflective color liquid crystal display devices are currently inferior to transmissive liquid crystal display devices, CRTs and the like in brightness, display quality, color reproductivity and the like. Therefore, the reflective color liquid crystal display devices capable of higher quality image representation are needed.
As one possible solution to this need, polymer-dispersed type liquid crystal (hereinafter referred to as twisted reversemode PDLC) has been proposed which is prepared by blending an acrylic monomer or oligomer having a photopolymerizing liquid crystal skelton (hereinafter referred to as liquid crystalline monomer and liquid crystalline oligomer respectively) with a host liquid crystal; injecting the blend into a cell that has undergone alignment process; twisting the blend in a specific orientation and finally subjecting the blend to UV irradiation to polymerize the liquid crystalline monomer or oligomer.
When a voltage is applied to the twisted reverse mode PDLC, the host liquid crystal changes its alignment orientation along the resultant electric field. However, the alignment orientation of the liquid crystalline polymer that has been polymerized and immobilized remains unchanged. As a result, mismatching in reflectivity occurs on the interface between the liquid crystal molecule and liquid crystalline polymer, leading to a light scattering phenomenon that features strong anisotropy in a particular orientation (see Japanese Unexamined Patent Application No. Hei 7(1995)-36022, and H. Kobayashi et al., Seiko Epson Corp., SID97, pp751-754).
Compared with conventional PDLCs that involve liquid crystal droplet structure, the above-mentioned twisted reverse mode PDLC has a greater proportion of liquid crystal component relative to liquid crystalline monomer or oligomer (usually, the approximate ratio of liquid crystalline monomer or oligomer to liquid crystal is 2:8 to 1:9), and, as a result, can be driven by a lower voltage. Capitalizing on this feature, a reflective color liquid crystal display device was proposed (see, T. Sonehara et al., Seiko Epson Corp., SID97, pp1023-1026).
A reflective color liquid crystal display device based on the above-mentioned twisted reverse mode PDLC mode is capable of bright image representation since it does not involve a polarizer. However, this device type is essentially incapable of black representation, and its image representation quality is greatly dependent on the surrounding lighting environments. More specifically, in a lighting environment where the main component of incident light is a light beam with strong directivity (for example, in the outdoors in a sunny day), the device provides relatively bright image representation of higher contrast; in contrast, its image representation quality significantly deteriorates in a lighting environment where light comes from all directions (for example, in the outdoors in a cloudy day, or in a room).
To remedy this drawback, a single-polarizer twisted reverse mode PDLC was proposed which contains an addition of one polarizer and one or more phase plates (see, Japanese Unexamined Patent Application No. Hei 7(1995)-218905). Basically, in this PDLC, when no voltage is applied to the device, liquid crystalline polymer molecules with liquid crystal skeltons and liquid crystal molecules are twisted in a common orientation, thereby the device remains transparent. In contrast, when a voltage exceeding a Freedericksz transition point is applied to the device, the liquid crystal molecules alone are oriented along the resultant electric field, causing mismatching in reflectivity between the liquid crystalline polymer molecules and the liquid crystal molecules, leading to light scattering.
The single polarizer and phase contrast films are arranged such that linearly polarized light incoming through the single polarizer is converted into circularly polarized light at a reflective plane owing to the phase plates as well as a birefringence effect of a composite layer formed of the twisted liquid crystal molecules and liquid crystalline polymer molecules (hereinafter referred to as twist PDLC layer). As a result of this arrangement, the linearly polarized light incoming through the polarizer is reflected as circularly polarized light from the reflection plane, and, when reaching the polarizer again, it is transformed into linearly polarized light with a plane of polarization that squarely intersects the transmission axis of the polarizer, and is absorbed, thereby the device becomes capable of black representation.
In contrast, when the liquid crystal molecules are oriented along the electric field owing to an applied voltage, linearly polarized light having passed the polarizer is scattered by the twist PDLC layer with the plane of polarization maintained before it is converted into circularly polarized light, and is further scattered by the reflective plane. Because the light is reflected by the reflective plane in a polarized state near linear polarization, it can again pass the polarizer, thereby device becomes capable of white representation. To attain satisfactory black and white representation using a reflective color liquid crystal display device of this configuration, it is necessary to design the twist PDLC layer in order to determine the retardation values (xcex94nxc2x7d=birefringence of liquid crystal xcex94nxc3x97layer thickness d) that allows the incoming linearly polarized light to be converted into circularly polarized light at the reflective plane with the device having a transparent state.
Incidentally, to realize brighter white representation, it is necessary to increase the difference in refractive index between the liquid crystal and liquid crystalline polymer so as to enhance light scattering performance. To improve black representation, it is necessary to decrease xcex94nxc2x7d or increase the twist angle of twist PDLC layer. The previously mentioned Japanese Unexamined Patent Application No. 218905/1995 discloses that the ideal combination for this purpose (xcex94n, d, twist angle) is (0.088, 2.3 xcexcm, 63 degrees) or (0.25, 2.2 xcexcm, 200 degrees).
However, a liquid crystal material with xcex94n=0.88 cannot achieve sufficient light scattering characteristics and brighter white representation. Thus, the similar material seems not to be readily used in practical application. When a liquid crystal material whose xcex94n=0.25 is used to realize bright white representation, the twist angle must be 200 degrees. A greater twist angle will, as shown later in Table 1 and FIG. 7 lead to loss in brightness when viewed in front of the display. Thus, this arrangement will have difficulty in providing brighter white representation. As described above, it is currently difficult to determine an optimal combination of conditions that satisfies not only xcex94nxc2x7d for achieving satisfactory black representation but also a greater xcex94n for attaining brighter white representation.
The present invention is intended to solve these problems. Therefore, it is the object of the invention to provide a reflective liquid crystal display device that realizes both brighter white representation and satisfactory black representation, by adopting a configuration that is capable of black representation, based on the optical rotating power of the twist PDLC layer.
To achieve the above-mentioned object, the present invention realizes black representation by using the rotary polarization where the plane of polarization of the linearly polarized light arriving at the twist PDLC layer rotates, rather than by based on the phase shift effect that occurs when incident light passes the twist PDLC layer.
The present invention provides a reflective liquid crystal display device comprising: first and second substrates opposed to each other; a liquid crystal layer sandwiched between the first and second substrates, the liquid crystal layer including a polymer-dispersed liquid crystal containing a liquid crystal and a liquid crystalline polymer; a transparent electrode interposed between the first substrate and liquid crystal layer; a quarter-wave plate interposed between the liquid crystal layer and second substrate; a reflective electrode interposed between the quarter-wave plate and second insulating substrate; and a polarizer disposed on the opposite side of the first substrate from the liquid crystal layer; wherein the liquid crystal and the liquid crystalline polymer are twisted in molecule alignment so as to have a twist angle X represented by X=(45xc2x0+90xc2x0 n)xc2x110xc2x0 (n=0 or a positive integer).